Positron emission tomography (PET) is a technology that is used to observe metabolic processes in the body. The PET technology has been widely used in medical diagnosis. The PET technology may use a plurality of detector modules that form a detector ring to detect pairs of gamma rays emitted indirectly by a positron-emitting radionuclide, which is introduced into the body on a biologically active molecule. It is desirable to provide a detector structure with simplicity of installation and maintenance.